On a semiconductor substrate, generally, transistor elements having gate insulating films different in film thickness for coping with various operating voltages are formed.
Further, there is a known LSI mounting flash memory elements in addition to the transistor elements in a mixed manner for the purpose of higher function.
Patent Document 1: Japanese Laid-open Patent Publication No. 2015-99892
Patent Document 2: Japanese Laid-open Patent Publication No. 2012-4403
To cope with particularly high voltage by different operating voltages, it is necessary to use a gate insulating film having a larger film thickness from the viewpoint of ensuring withstand voltage. In the case of various operating voltages, it is necessary to prepare gate insulating films having different thicknesses for the various operating voltages, respectively.
Besides, for operation of the flash memory element, a high voltage of 5 V or higher is necessary. In the case where the gate insulating film for a high withstand voltage transistor is formed to cope with the high voltage, not only the number of steps is increased but also a step accompanied by high heat such as gate oxidation is further added. The step accompanied by heat may affect characteristics of other low-voltage transistor elements.